1. Field of the Invention
The present invention relates to a digital transmission system, and more particularly to a measure against faults in a digital transmission system extending via a plurality of transmission control units.
2. Description of the Prior Art
In order to cope with faults such as noise interference and disconnection in a digital transmission system, various mechanisms have heretofore been proposed and put into practical use. However, a large number of points to be improved are left in taking measures against faults in a high-speed digital transmission system in which data is relayed via a plurality of transmission control units. One prior effort described in Japanese Patent Application Laid-open No. 57-56060, with the intention of coping with a transient fault in a loop transmission system, circulates test data between two transmission control units and measures the frequency of occurrence of abnormal data so as to discriminate a cause of the fault and performs the report of the abnormal situation, the reconstruction of transmission paths, etc. as may be needed.
With this method, however, in a case where a plurality of transmission control units intervene, it is difficult to satisfactorily localize and find a faulty position. Moreover, when a large number of abnormalities have continuously occurred for such a reason that a single data block is apparently divided into a plurality of data blocks by the fault, the transmission control unit is very busy with the processing of the abnormalities, and the transmission of normal data might be adversely affected.